<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        body{
          width: 100%;
          margin: o auto;
          height: 300px;
          position: fixed;
        }
        table {
            width: 500px;
            height: auto;
            position: relative;
            left:50%;
            margin-left: -250px;
            justify-content: center;
            align-content: center;
            align-items: center;
        }
        tr{
          display: flex;
          justify-content: center;
          align-content: center;
          align-items: center;
          border-bottom: 1px solid #333;
          text-align: center;
        }
        td,
        th {
            flex:1;
        }
        
        thead tr {
            height: 40px;
            background-color: #ccc;
        }
    </style>
</head>
<body>
    <table cellspacing="0">
        <thead>
          <tr>
            <th>姓名</th>
            <th>科目</th>
            <th>成绩</th>
            <th>操作</th>
          </tr>
        </thead>
        <tbody>
        </tbody>
      </table>
</body>
</html>